1. Field of the Invention
The invention regards an outlet arrangement for use in separation of water and oil by means of a two-phase liquid/liquid separator in which the oil and water phases are respectively separated over at least part of the length of the separator from an upstream end to a downstream end of the separator. In particular, the invention regards an outlet arrangement for a horizontal gravity separator with a high length/diameter ratio. The invention especially regards a downhole separator for placing in an underground well. The invention also regards a method for inhibiting re-mixing of the oil and water phases in a separator where the separator includes a separator chamber with an upstream end and a downstream end and an oil outlet and a water outlet, the water outlet being situated in an upper part of the separator chamber and the oil outlet being situated in a lower part of the separator chamber.
The invention also regards a method for orienting an outlet arrangement in a horizontal downhole separator, as well as means of implementing this orientation.
2. Description of the Related Art
Separation of well fluids is usually carried out by use of a 3-phase gravity separator on a platform. Typically, the vessels have a diameter/length ratio in the range of 1:3 to 1:5. The separator is usually partially filled with gas, so as to provide two interfaces; a gas-liquid interface and an oil-water interface. Water and oil are extracted by means of a suitably elevated, vertically oriented pipe with a vortex breaker. The pipes are typically provided in the lower part of the vessel, at the opposite end from the fluid inlet. A weir plate is often used to isolate the water phase from the oil outlet. In this case, the oil-water interface is kept below the level of the weir plate, so that only the oil phase will flow over this.
In a horizontal tube or a vessel, the oil and water are separated by gravity due to the difference in density between the two phases, so as to form a lighter phase and a heavier phase. A stratified flow pattern must be maintained in order to be able to separate the liquid phases. The cross-sectional flow area is sized with respect to fluid velocity and water cut. If the residence time is sufficiently high to transfer droplets from the continuous phases, clean oil and water can be produced. The oil droplets will rise from the water phase, and if given sufficient residence time, they will reach the oil-water interface. Water droplets in the oil phase will settle and eventually reach the interface. Accumulated droplets of oil and water will form a dense layer in the interfacial region, in which coalescence takes place. If the residence time is sufficient, the dense layer will eventually be broken up.
Gas may be separated out from the well fluid by use of one or more cyclone separators. The water may also be cleaned by use of hydrocyclones. In a multi-stage separation system consisting of such separators, the main aim is to provide a clean oil phase. This is best achieved by employing a long, slim separator geometry in order to maximise the oil-water interface where the coalescence of the droplets takes place. Furthermore, the distance to the oil-water interface, across which distance the droplets have to travel, is reduced.
A downhole oil/water gravity separator in which the separation takes place in a section of the production tubing in a horizontal well, such as disclosed in WO 98/41304, is a separator with a very high length to diameter ratio. A separator of this type will typically have a length of approximately 100 m, while the diameter is the same as for the production tubing, for instance 9″, 10¾″ or 13″, i.e. a length-diameter ratio of 1:300 to 1:400.
A large oil-water interface and sufficient residence time alone is not enough to achieve clean oil and water phases at the outlet of the separator. It is crucial that the separation of oil and water take place without either of the phases becoming contaminated by the other upon extraction from the separator.
Since the diameter of the outlet is small, this will easily influence the oil-water interface and thereby result in water entering the oil phase and oil entering the water phase.